I don’t know how many of those mass shootings, were with guns locally purchased in state or obtained out of state.I believe California has the strictest gun control in the US but also the highest number of mass shootings.
No amount of strict state laws will help if the neighbouring states have weaker gun laws. A sick individual that cannot legally obtain a gun in California might be able to get one in Nevada with no issues. But I don’t know if that is a thing or not.
Florida and Texas have the next highest amount of mass shootings and their laws are not as strict. Texas also having the highest number of guns of any state.
I’m not sure gun laws being strict or loose are a metric that should be used one way or another. I’ve said this before, the US has a gun culture historically ingrained in its DNA from the time it was created as a nation. Not sure if that problem will ever be solved.